At small contacted gate pitch, overlay tolerances are an unacceptably large portion of the layout footprint for source/drain contact design, leading to a need for self-aligned source/drain contacts. Current methods for achieving a self-aligned source/drain contact in a replacement gate flow involve timed etch-back of a filled metal gate stack followed by a dielectric fill.
However, state-of-the-art metal gate stacks typically include many different materials such as, for example, a high-k dielectric, workfunction setting materials, workfunction modification materials, and conductive filler materials. As a result, uniform etch-back of this multi-material gate stack can be very difficult to achieve, thereby increasing the risk of shorting between source/drain contacts and gate stacks.